Semiconductor RF-switches (Radio Frequency and Micro-wave Switches), such as a HEMT switch, an MESFET switch, and a PIN diode switch which use GaAs substrate, are currently the mainstream of RF-switches.
However, to achieve higher performance and lower power consumption of radio terminals, it has been proposed to utilize a device using microelectromechanical elements in addition to conventional semiconductor elements.
This device is an electromechanical switch adapted to drive microelectrodes by an electrostatic force or the like and to mechanically control the relative distance between the electrodes thereby to perform the turn-on or the turn-off of signals. At the turn-on, the electrodes are electrically in contact with each other. Therefore, the loss between the electrodes is extremely small and a low-loss switch can be realized.
Especially, an RF-switch applied to the front end portion of a radio terminal requires low loss and low power consumption. Such a device using microelectromechanical elements is expected as a useful resolution method.
Many kinds of switches using conventional electromechanical elements have been devised. Non-patent Document 1 covers most of such switches.
For example, a switch using RFMEMS (Radio Frequency Microelectromechanical Systems) described in Non-patent Document 1 is constituted by one movable electrode and one fixed electrode. When a DC voltage is applied between the movable electrode and the fixed electrode as a drive control voltage, an electrostatic force is generated. The movable electrode is pulled in toward the fixed electrode using the electrostatic force as a driving force. The electrodes are physically in contact with each other. An input signal inputted from a movable-electrode-side input terminal is outputted to a fixed-electrode-side output terminal, so that signals are coupled.
A method of coupling signals includes a method of bringing metal into direct contact with metal and a method of capacitively coupling metals through an insulator. Either of the methods can realize low-loss coupling.
When the drive control voltage applied between the electrodes is changed to 0, the electrostatic force is canceled. The movable electrode is returned to an initial position, utilizing a spring force thereof as a driving force. At that time, the distance between the movable electrode and the fixed electrode is sufficiently large. Thus, the capacitance value between the electrodes is small. Consequently, no capacity coupling therebetween occurs. Signals to be coupled between the electrodes can be shielded.
Thus, when the distance between the electrodes is sufficiently large, the isolation therebetween can sufficiently be ensured. Also, the loss is extremely small. This switch excels in electrical properties, as compared with the RF switch using conventional semiconductors.
Also, an MEMS switch of this kind has been proposed by Patent Document 1.
An object of the MEMS switch described in Patent Document 1 is to reduce a response time and an application voltage. This switch has first, second, and third beams arranged to be spaced slightly distant, and voltage applying means adapted to apply an electrostatic force to the beams. This switch is configured so that the position of each of the beams and the capacity between the beams are changed by the electrostatic force. Both of the first beam and the second beam are moved, so that the beams can electrically be coupled together at high speed. Also, to put off the beams at high speed, an electrostatic force is caused on the third beam that faces the second beam and that is preliminarily placed close to the first beam and the second beam. Consequently, a strong electrostatic force can be applied between the second beam and the third beam. Thus, this switch makes a response at higher speed.
Additionally, a same-curve-shaped part is provided in each of the beams. This can alleviate change in a pull-in voltage, which corresponds to change in the internal stress of the beam, and also can alleviate change in the beam-to-beam capacitance due to beam strain.
Non-Patent Document 1: Gabriel M. Rebeiz, “RF MEMS THEORY, DESIGN, AND TECHNOLOGY”, John Wiley & Sons, Feb. 1, 2003, p. 122.
Patent Document 1: JP-A-2004-111360 (pages 5 and 6, FIG. 1, and FIG. 3(a) to FIG. 3(f)).